dipper@utastro.UUCP (Debbie Byrd) (04/05/85)
People who live near the ocean may see some especially high tides today. More on why -- after this. April 5 Spring Tides You may have heard the expression "spring tides." Well, this is spring -- and we may have some spring tides along the coastlines of our planet Earth today. But actually these tides have nothing to do with the season. The expression spring tides comes from the root word springen which means "to leap up." A spring tide can come at any time of year. It's a especially high tide -- caused by the sun and the moon. In general, the moon's gravity causes the tides. The closer the moon is to the Earth, the harder its gravity pulls. The moon's orbit isn't perfectly circular -- so once each month the moon reaches a closest point to our world -- called the moon's perigee -- a word that actually means "near the Earth." It so happens that the moon is at perigee today -- and that fact contributes to today's high spring tides. But there's something else causing the high tides -- the fact that the moon also reaches the crest of its full phase today. You may have noticed a very full looking moon in the sky last night. It'll be there again when the sun goes down Friday evening. A full moon at perigee is a prescription for very high spring tides -- because a full moon is opposite the sun in space -- or lined up in space with the Earth and sun. During such a time, the gravity of the sun comes into play, too -- causing tides that are higher than usual -- which some of you who live near the ocean may notice. Script by Deborah Byrd. (c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin
crs@lanl.ARPA (04/08/85)
> > It so happens that the moon is at perigee today -- and that fact > contributes to today's high spring tides. But there's something else > causing the high tides -- the fact that the moon also reaches the crest > of its full phase today. You may have noticed a very full looking moon > in the sky last night. It'll be there again when the sun goes down > Friday evening. > > A full moon at perigee is a prescription for very high spring tides -- > because > > a full moon is *opposite* the sun in space -- or lined up in > space with the Earth and sun. > > During such a time, the gravity of the > sun comes into play, too -- causing tides that are higher than usual -- > which some of you who live near the ocean may notice. > Am I missing something? If, as described above, the earth, moon and sun are in a line with earth in the middle, won't the gravitational fields of moon and sun subtract rather than add? No flames, please. I'm not an astronomer, just interested. Charlie Sorsby ...!{cmcl2,ihnp4,...}!lanl!crs crs@lanl.arpa
daemon@decwrl.UUCP (The devil himself) (04/10/85)
---
To the person confused about high tides being caused by the sun and
moon being on opposite sides of the earth.
If we were to put you on a rack and start stretching, the center of
your body would not move because the forces would counteract each
other. However, you would become longer.
Vick Bennison
...decvax!decwrl!rhea!tools!bennison
(603) 881-2156bob@islenet.UUCP (Robert P. Cunningham) (04/10/85)
> Am I missing something? If, as described above, the earth, moon and > sun are in a line with earth in the middle, won't the gravitational > fields of moon and sun subtract rather than add? Tidal forces are the resultant difference between gravitational attraction and centrifugal forces. At the surface of the Earth, around the point directly underneath the Moon, the gravitational attraction is stronger. The net effect is to create a bulge pointing towards the Moon. At the antipode, on the other side from the Moon, centrifugal forces are stronger than the Moon's gravitational pull. The net effect is to create another bulge, pointing away from the Moon. [yes, this seems counter-intuitive] Thus, you get a bulges (high tides) point both towards and away from the Moon. This is the major tide cycle, with a period of 1/2 a lunar day (12 hours, 25.235 minutes). The tidal potential due to the Sun creates two similar bulges and a tidal cycle of 1/2 the solar day. When these are in line (both Sun & Moon on the same side of the Earth, or on opposite sides), that's when the highest tides occur. Important secondary effects influencing the tidal forces involve the relative declinations of the Sun & Moon, and the actual current distance of the Moon from Earth. The actual resultant rise (or fall) in sea level depends greatly on the local configuration of the ocean basin and shoreline configuration. And, of course, there are tidal effects in the Earth's mantle and atmosphere, not just the oceans. In fact, there are measurable tidal effects even in various lakes. -- Bob Cunningham ..{dual,ihnp4,vortex}!islenet!bob Honolulu, Hawaii